Even though I agree that most of the CMYK cases mentioned use CMYK
almost as spot colors, I can think of a very common usage scenario in
Graphic Design where you need to be able to edit CMYK directly:
Most frequently Pantones. Brands have their corporate colors and ask
designers to use them, but they can not always afford extra spot passes
in offset press, so the colors have to be converted to the most
aproximate CMYK combination (the Pantone Bridge catalog is for that).
So you have to adjust the color of a photograph of a sign, a truck and a
producto of your client to their corporate CMYK color.
It's a photograph, you need CMYK, you can't use spot.
This is a very common scenario, and it's a task for a image manipulation
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